Endocrine System: Thyroid, Parathyroid, and Thymus

Parathyroid Gland

• Small, rounded mass attached to the posterior surface of the thyroid gland
• Produces Parathyroid hormone (PTH) which controls homeostasis of calcium and phosphate in blood by activating Vitamin D

Thymus

• Plays a role in the immune system
• Produces thymosin, thymic humoral factor, and thymic factor
• Responsible for the maturation of T-lymphocytes

Thyroid, Parathyroid, and Thymus Hormones

• Thyroid gland produces T3 and T4, which increase energy utilization, oxygen consumption, and growth and development
• Thyroid gland produces calcitonin, which decreases calcium ion concentration in body fluids
• Parathyroid gland produces PTH, which increases calcium ion concentration in body fluids
• Thymus produces thymosin, which is responsible for the maturation and functional competence of the immune system

Adrenal Gland

• Located superior to the kidney
• Divided into the outer cortex and inner medulla
• Adrenal cortex produces steroid hormones, including glucocorticoids, mineralocorticoids, and gonadocorticoids
• Adrenal medulla produces hormones controlled by the ANS, including epinephrine and norepinephrine

Adrenal Hormones

• Mineralocorticoids (e.g., aldosterone) increase renal reabsorption of sodium ions and water, and decrease renal potassium ion loss

Pituitary Gland

• Divided into the neurohypophysis (posterior lobe) and adenohypophysis (anterior lobe)
• Neurohypophysis stores hormones, including oxytocin and anti-diuretic hormone (ADH)
• Adenohypophysis releases hormones, including growth hormone (GH), thyroid-stimulating hormone (TSH), adrenocorticotropic hormone (ACTH), luteinizing hormone (LH), follicle-stimulating hormone (FSH), melanocyte-stimulating hormone (MSH), and prolactin (PRL)

Pituitary Hormones

• Oxytocin stimulates contraction of the uterus and contractile cells of the breast
• ADH prevents excess urine production
• GH promotes general body growth
• ACTH stimulates the adrenal cortex to release hormones
• TSH controls the thyroid gland
• LH and FSH stimulate sexual and reproductive function
• MSH is related to skin pigmentation
• PRL initiates milk production in the breast

Thyroid Gland

• Located in the middle anterior part of the neck
• Produces thyroxin (T4) and tri-iodothyronine (T3)
• Calcitonin is involved in calcium and phosphate homeostasis
• Rich blood supply allows for the delivery of high levels of hormones in a short period of time

Endocrine Disorders

• Acromegaly: overproduction of growth hormone after the epiphyseal plates have fused, leading to bone shape changes, broad facial features, and an enlarged lower jaw
• Cretinism: severely stunted physical and mental growth due to thyroid hormone insufficiency in infancy
• Goiter: enlarged thyroid gland due to thyroid hormone hyposecretion, often caused by iodine insufficiency
• Cushing's syndrome: results from hypersecretion of glucocorticoids, leading to lipid reserves being mobilized, adipose tissue accumulating in the cheeks and base of the neck, and other effects

Prostaglandins

• Regulate cell metabolism
• Important in normal physiology of smooth muscle, blood flow, platelet function, nerve transmission, immune response, secretion, reproduction, and respiration
• Induce inflammation, promote fever, and intensify pain

Feedback Control of Hormone Release

• Regulation involves the production of releasing substances or inhibiting substances
• Involves the release of hormone from the pituitary gland, stimulation of hormone release by the target gland, and the hormone exerting its normal effect and influencing further release of a releasing or inhibiting substance

Testes and Ovaries

• Testes: located within the scrotum, produce testosterone, and stimulate the development of male sexual characteristics
• Ovaries: located in the pelvic cavity, produce oestrogen and progesterone, and are responsible for the development and maintenance of female characteristics and the menstrual cycle

Hormones of the Reproductive System

• Testosterone: involved in the maturation of sperm and male secondary sex characteristics
• Inhibin: inhibits the secretion of FSH
• Oestrogen: involved in follicle maturation and female secondary sex characteristics
• Progesterone: prepares the uterus for implantation and prepares the breast for secretory function

Endocrine System

• The endocrine system is a network of ductless glands that produce and secrete hormones that regulate various bodily functions.
• The system consists of major endocrine glands, including the hypothalamus, pituitary gland, thyroid gland, parathyroid gland, thymus, adrenal gland, pancreas, kidneys, heart, digestive tract, placenta, testes, ovaries, and pineal gland.

Hypothalamus

• The hypothalamus is the "master gland" that controls the body's temperature, hunger, and thirst, and regulates hormone secretion.
• It receives information from the body and interprets and evaluates it, sending outgoing messages via nerves or hormones.

Pituitary Gland

• The pituitary gland is a pea-sized mass of glandular tissue located in the sella turcica, connected to the hypothalamus by a slender stalk called the infundibulum.
• It has two parts: the neurohypophysis and adenohypophysis, and indirectly controls growth, metabolism, sexual reproduction, and lactation.

Adrenal Gland

• The adrenal gland is divided into the outer cortex and inner medulla.
• The adrenal cortex secretes steroid hormones, including glucocorticoids, mineralocorticoids, and gonadocorticoids.
• The adrenal medulla secretes hormones controlled by the autonomic nervous system, including epinephrine (adrenaline) and norepinephrine (noradrenaline).

Pancreas

• The pancreas is a flattened organ located retroperitoneally and transversely across the posterior abdominal wall.
• It is classified as both exocrine and endocrine, with the islets of Langerhans secreting glucagon, insulin, and growth hormone-inhibiting hormone.

Parathyroid Gland

• The parathyroid gland is a small, rounded mass attached to the posterior surface of the thyroid gland.
• It produces parathyroid hormone (PTH), which controls the homeostasis of calcium and phosphate in the blood by activating vitamin D.

Thymus

• The thymus plays a role in the immune system, producing thymosin, thymic humoral factor, and thymic factor.
• It is responsible for the maturation of T-lymphocytes.

Thyroid Gland

• The thyroid gland is located in the neck and produces triiodothyronine (T3) and thyroxine (T4), which regulate energy utilization, growth, and development.
• It also produces calcitonin, which decreases calcium ion concentration in body fluids.

Adrenal Hormones

• The adrenal cortex secretes mineralocorticoids (e.g., aldosterone), glucocorticoids (e.g., cortisol), and gonadocorticoids (e.g., estrogen and androgens).
• The adrenal medulla secretes epinephrine (adrenaline) and norepinephrine (noradrenaline), which increase cardiac activity, blood pressure, and glycogen breakdown, and mobilize lipid reserves.

Reproductive System Hormones

• Testes produce testosterone, which stimulates the development of male sexual characteristics.
• Ovaries produce estrogen and progesterone, which are responsible for female sexual characteristics and the menstrual cycle.

Other Endocrine Tissues

• The kidneys produce erythropoietin, which stimulates the production of red blood cells.
• The heart produces atrial natriuretic factor (ANF), a powerful vasodilator hormone.